During manufacture of integrated circuits, a wide variety of operating characteristics and circuit functions are tested. Many integrated circuits include test circuitry which is enabled during testing operations and disabled during normal integrated circuit operations. For example, normal operations in a 1M.times.4 memory chip include reading and writing four data bits at a time. To improve testing efficiency, test circuitry may be integrated within the memory chip to test 64 data bits at a time. One way of enabling such test circuitry is to apply to an externally accessible terminal an elevated voltage not expected during normal memory chip operations (commonly known as a supervoltage). The memory chip includes a circuit known as a supervoltage circuit which detects the supervoltage and responsively enables the test circuitry.
Because integrated circuit fabrication involves a number of process steps, variations in circuit element parameters are commonplace. For example, significant variations in transistor threshold voltages can occur, which, if large enough, cause the integrated circuit to malfunction. Even variations that do not deleteriously affect intended integrated circuit function can nevertheless affect the testing of that integrated circuit function. In particular, high transistor threshold voltages can result in supervoltage magnitudes sufficient to cause transistor junction breakdown. This is especially true for those tests in which the semiconductor substrate is negatively biased.